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# PushbackInputStream

`public class PushbackInputStream`
`` `` `extends FilterInputStream` ``

java.lang.Object
↳ | java.io.InputStream
| ↳ | java.io.FilterInputStream
|  | ↳ | java.io.PushbackInputStream

A `PushbackInputStream` adds functionality to another input stream, namely the ability to "push back" or "unread" one byte. This is useful in situations where it is convenient for a fragment of code to read an indefinite number of data bytes that are delimited by a particular byte value; after reading the terminating byte, the code fragment can "unread" it, so that the next read operation on the input stream will reread the byte that was pushed back. For example, bytes representing the characters constituting an identifier might be terminated by a byte representing an operator character; a method whose job is to read just an identifier can read until it sees the operator and then push the operator back to be re-read.

## Summary

### Fields
`protected byte[]` | `buf` The pushback buffer.
`protected int` | `pos` The position within the pushback buffer from which the next byte will be read.

### Inherited fields
From class `java.io.FilterInputStream`
`protected InputStream` | `in` The input stream to be filtered.

### Public constructors
`PushbackInputStream(InputStream in, int size)` Creates a `PushbackInputStream` with a pushback buffer of the specified `size`, and saves its argument, the input stream `in`, for later use.
`PushbackInputStream(InputStream in)` Creates a `PushbackInputStream` and saves its argument, the input stream `in`, for later use.

### Public methods
`int` | `available()` Returns an estimate of the number of bytes that can be read (or skipped over) from this input stream without blocking by the next invocation of a method for this input stream.
`void` | `close()` Closes this input stream and releases any system resources associated with the stream.
`void` | `mark(int readlimit)` Marks the current position in this input stream.
`boolean` | `markSupported()` Tests if this input stream supports the `mark` and `reset` methods, which it does not.
`int` | `read()` Reads the next byte of data from this input stream.
`int` | `read(byte[] b, int off, int len)` Reads up to `len` bytes of data from this input stream into an array of bytes.
`void` | `reset()` Repositions this stream to the position at the time the `mark` method was last called on this input stream.
`long` | `skip(long n)` Skips over and discards `n` bytes of data from this input stream.
`void` | `unread(byte[] b, int off, int len)` Pushes back a portion of an array of bytes by copying it to the front of the pushback buffer.
`void` | `unread(int b)` Pushes back a byte by copying it to the front of the pushback buffer.
`void` | `unread(byte[] b)` Pushes back an array of bytes by copying it to the front of the pushback buffer.

### Inherited methods
From class `java.io.FilterInputStream`
`int` | `available()` Returns an estimate of the number of bytes that can be read (or skipped over) from this input stream without blocking by the next caller of a method for this input stream.
`void` | `close()` Closes this input stream and releases any system resources associated with the stream.
`void` | `mark(int readlimit)` Marks the current position in this input stream.
`boolean` | `markSupported()` Tests if this input stream supports the `mark` and `reset` methods.
`int` | `read()` Reads the next byte of data from this input stream.
`int` | `read(byte[] b, int off, int len)` Reads up to `len` bytes of data from this input stream into an array of bytes.
`int` | `read(byte[] b)` Reads up to `byte.length` bytes of data from this input stream into an array of bytes.
`void` | `reset()` Repositions this stream to the position at the time the `mark` method was last called on this input stream.
`long` | `skip(long n)` Skips over and discards `n` bytes of data from the input stream.
From class `java.io.InputStream`
`int` | `available()` Returns an estimate of the number of bytes that can be read (or skipped over) from this input stream without blocking by the next invocation of a method for this input stream.
`void` | `close()` Closes this input stream and releases any system resources associated with the stream.
`void` | `mark(int readlimit)` Marks the current position in this input stream.
`boolean` | `markSupported()` Tests if this input stream supports the `mark` and `reset` methods.
`abstract int` | `read()` Reads the next byte of data from the input stream.
`int` | `read(byte[] b, int off, int len)` Reads up to `len` bytes of data from the input stream into an array of bytes.
`int` | `read(byte[] b)` Reads some number of bytes from the input stream and stores them into the buffer array `b`.
`void` | `reset()` Repositions this stream to the position at the time the `mark` method was last called on this input stream.
`long` | `skip(long n)` Skips over and discards `n` bytes of data from this input stream.
From class `java.lang.Object`
`Object` | `clone()` Creates and returns a copy of this object.
`boolean` | `equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void` | `finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>` | `getClass()` Returns the runtime class of this `Object`.
`int` | `hashCode()` Returns a hash code value for the object.
`final void` | `notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void` | `notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String` | `toString()` Returns a string representation of the object.
`final void` | `wait(long timeout, int nanos)` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed.
`final void` | `wait(long timeout)` Causes the current thread to wait until either another thread invokes the `notify()` method or the `notifyAll()` method for this object, or a specified amount of time has elapsed.
`final void` | `wait()` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object.
From interface `java.io.Closeable`
`abstract void` | `close()` Closes this stream and releases any system resources associated with it.
From interface `java.lang.AutoCloseable`
`abstract void` | `close()` Closes this resource, relinquishing any underlying resources.

## Fields

### buf

    protected byte[] buf

The pushback buffer.

### pos

    protected int pos

The position within the pushback buffer from which the next byte will be read. When the buffer is empty, `pos` is equal to `buf.length`; when the buffer is full, `pos` is equal to zero.

## Public constructors

### PushbackInputStream

    public PushbackInputStream (InputStream in,
                    int size)

Creates a `PushbackInputStream` with a pushback buffer of the specified `size`, and saves its argument, the input stream `in`, for later use. Initially, there is no pushed-back byte (the field `pushBack` is initialized to `-1`).

Parameters
`in` | `InputStream`: the input stream from which bytes will be read.
`size` | `int`: the size of the pushback buffer.

Throws
`IllegalArgumentException` | if `size <= 0`

### PushbackInputStream

    public PushbackInputStream (InputStream in)

Creates a `PushbackInputStream` and saves its argument, the input stream `in`, for later use. Initially, there is no pushed-back byte (the field `pushBack` is initialized to `-1`).

Parameters
`in` | `InputStream`: the input stream from which bytes will be read.

## Public methods

### available

    public int available ()

Returns an estimate of the number of bytes that can be read (or skipped over) from this input stream without blocking by the next invocation of a method for this input stream. The next invocation might be the same thread or another thread. A single read or skip of this many bytes will not block, but may read or skip fewer bytes.

The method returns the sum of the number of bytes that have been pushed back and the value returned by `available`.

Returns
`int` | the number of bytes that can be read (or skipped over) from the input stream without blocking.

Throws
`IOException` | if this input stream has been closed by invoking its `close()` method, or an I/O error occurs.

See also:

- `FilterInputStream.in`
- `InputStream.available()`

### close

    public void close ()

Closes this input stream and releases any system resources associated with the stream. Once the stream has been closed, further read(), unread(), available(), reset(), or skip() invocations will throw an IOException. Closing a previously closed stream has no effect.

Throws
`IOException` | if an I/O error occurs.

### mark

    public void mark (int readlimit)

Marks the current position in this input stream.

The `mark` method of `PushbackInputStream` does nothing.

Parameters
`readlimit` | `int`: the maximum limit of bytes that can be read before the mark position becomes invalid.

See also:

- `InputStream.reset()`

### markSupported

    public boolean markSupported ()

Tests if this input stream supports the `mark` and `reset` methods, which it does not.

Returns
`boolean` | `false`, since this class does not support the `mark` and `reset` methods.

See also:

- `InputStream.mark(int)`
- `InputStream.reset()`

### read

    public int read ()

Reads the next byte of data from this input stream. The value byte is returned as an `int` in the range `0` to `255`. If no byte is available because the end of the stream has been reached, the value `-1` is returned. This method blocks until input data is available, the end of the stream is detected, or an exception is thrown.

This method returns the most recently pushed-back byte, if there is one, and otherwise calls the `read` method of its underlying input stream and returns whatever value that method returns.

Returns
`int` | the next byte of data, or `-1` if the end of the stream has been reached.

Throws
`IOException` | if this input stream has been closed by invoking its `close()` method, or an I/O error occurs.

See also:

- `InputStream.read()`

### read

    public int read (byte[] b,
                    int off,
                    int len)

Reads up to `len` bytes of data from this input stream into an array of bytes. This method first reads any pushed-back bytes; after that, if fewer than `len` bytes have been read then it reads from the underlying input stream. If `len` is not zero, the method blocks until at least 1 byte of input is available; otherwise, no bytes are read and `0` is returned.

Parameters
`b` | `byte`: the buffer into which the data is read.
`off` | `int`: the start offset in the destination array `b`
`len` | `int`: the maximum number of bytes read.

Returns
`int` | the total number of bytes read into the buffer, or `-1` if there is no more data because the end of the stream has been reached.

Throws
`NullPointerException` | If `b` is `null`.
`IndexOutOfBoundsException` | If `off` is negative, `len` is negative, or `len` is greater than `b.length - off`
`IOException` | if this input stream has been closed by invoking its `close()` method, or an I/O error occurs.

See also:

- `InputStream.read(byte[], int, int)`

### reset

    public void reset ()

Repositions this stream to the position at the time the `mark` method was last called on this input stream.

The method `reset` for class `PushbackInputStream` does nothing except throw an `IOException`.

Throws
`IOException` | if this method is invoked.

See also:

- `InputStream.mark(int)`
- `IOException`

### skip

    public long skip (long n)

Skips over and discards `n` bytes of data from this input stream. The `skip` method may, for a variety of reasons, end up skipping over some smaller number of bytes, possibly zero. If `n` is negative, no bytes are skipped.

The `skip` method of `PushbackInputStream` first skips over the bytes in the pushback buffer, if any. It then calls the `skip` method of the underlying input stream if more bytes need to be skipped. The actual number of bytes skipped is returned.

Parameters
`n` | `long`:

Returns
`long` | the actual number of bytes skipped.

Throws
`IOException` | if the stream does not support seek, or the stream has been closed by invoking its `close()` method, or an I/O error occurs.

See also:

- `FilterInputStream.in`
- `InputStream.skip(long)`

### unread

    public void unread (byte[] b,
                    int off,
                    int len)

Pushes back a portion of an array of bytes by copying it to the front of the pushback buffer. After this method returns, the next byte to be read will have the value `b[off]`, the byte after that will have the value `b[off+1]`, and so forth.

Parameters
`b` | `byte`: the byte array to push back.
`off` | `int`: the start offset of the data.
`len` | `int`: the number of bytes to push back.

Throws
`IOException` | If there is not enough room in the pushback buffer for the specified number of bytes, or this input stream has been closed by invoking its `close()` method.

### unread

    public void unread (int b)

Pushes back a byte by copying it to the front of the pushback buffer. After this method returns, the next byte to be read will have the value `(byte)b`.

Parameters
`b` | `int`: the `int` value whose low-order byte is to be pushed back.

Throws
`IOException` | If there is not enough room in the pushback buffer for the byte, or this input stream has been closed by invoking its `close()` method.

### unread

    public void unread (byte[] b)

Pushes back an array of bytes by copying it to the front of the pushback buffer. After this method returns, the next byte to be read will have the value `b[0]`, the byte after that will have the value `b[1]`, and so forth.

Parameters
`b` | `byte`: the byte array to push back

Throws
`IOException` | If there is not enough room in the pushback buffer for the specified number of bytes, or this input stream has been closed by invoking its `close()` method.

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